The invention relates to a hydraulic brake system which is normally actuatable by means of an external force but in an emergency is actuatable by muscle power.
One such brake system is known from German Patent Disclosure DE 43 22 182 Al. The known brake system has an external energy source in the form of a hydraulic pump, which is drivable by an electric pump motor and pumps into a hydraulic pressure reservoir that contains brake fluid under pressure for braking. Connected to the hydraulic reservoir are wheel brake cylinders, and a wheel brake pressure in the wheel brake cylinders can be adjusted by means of wheel brake pressure control valve assemblies. These wheel brake pressure control valve assemblies typically have one brake pressure buildup valve and one brake pressure reducing valve for each wheel brake cylinder, or these two valves per wheel brake cylinder can be combined into a single valve.
As a brake force set-point value transducer for braking operation using an external force, the known brake system has a brake pedal and a conventional master cylinder, which for external force braking can be disconnected from the wheel brake cylinders by means of a downstream disconnection valve. The wheel brake pressures are adjusted as a function of a brake pedal travel or the actuation force for the brake pedal or the master cylinder pressure generated; different pressures can be established in the wheel brake cylinders in the front and rear, and in the case of brake slip control on the right and left as well.
The master cylinder of the known brake system is preferably a tandem master cylinder, of whose two positive displacement chambers at least one is assigned to front-wheel brake cylinders. If the external energy should fail, emergency braking by muscle power can be executed.
The positive displacement chambers of the tandem master cylinder are embodied with a closed disconnection valve in order to achieve piston travel upon external force braking: Brake fluid contained in one of the positive displacement chambers is positively displaced from it, by actuation of the master cylinder, into a brake fluid supply container, so that piston strokes are possible with the disconnection valve closed. A simulator spring inserted into one of the positive displacement chambers exerts forces on the pistons that increase linearly with the piston travel distances, so that a driver must exert a greater actuating force for a longer brake pedal travel. The increase may be linear by means of a linear simulator spring, or progressive.